Caterpillars cannot bite, but some sting, and the hairs that they release through shedding may cause allergic reactions in some individuals. Caterpillars' mouths lack the strength to pierce human skin, but they have other methods of defense. Some species have spines covering their skin that attach to poison glands and can secrete poison when caterpillars feel threatened.
As with other insects, there are many different species of caterpillars. Some are benign and pose little threat to humans while others contain powerful toxins. Some stinging caterpillars are the Saddleback, Puss, Io moth, Hag, Buck moth and Flannel moth. Saddlebacks have brown and green bodies that are cylindrical in shape and may reach 1 inch in length. Puss caterpillars are light brown in color and may grow to 1 inch in size. They have toxins in their hairs that produce pain in humans. Io moths have light green coats with stripes. Their long spines project outward. Hag caterpillars are a rusty brown and have horn-shaped spines. Buck moths vary in color from dark yellow and brown to purple. They have sharp stingers protruding from their spines. Flannel moths, which are white, have soft hairs of varying lengths; some contain toxins while others do not.
In addition to touching caterpillars, people may experience adverse reactions after inhaling caterpillar hairs or eating the insects. Symptoms range from headache and itching to gastrointestinal distress, severe pain, respiratory problems and hives.